Communication Method and Radio Transmitter

ABSTRACT

Radio transmission is performed even to a communication party whose bandwidth that can be used for transmission and reception is limited without having an influence of an offset of a DC component. A radio transmitter applied to an OFDMA communication system in which a plurality of different terminals performs communication using OFDM signals at the same time that includes a mapping part that allocates transmission power to each subcarrier, and also selects a subcarrier to which minimum power of the transmission power to be allocated is allocated and modulates transmission data in units of communication slots to output the modulated data; and a transmission part for transmitting radio signals including the modulated data using each of the subcarriers.

TECHNICAL FIELD

The present invention relates to a communication method and a radiotransmitter that perform radio transmission with a multi-carriertransmission system using communication slots.

BACKGROUND ART

In recent years, standardization for realizing broadband wirelessInternet access targeting a transmission rate of 10 Mbps to 100 Mbps hasbeen promoted and various kinds of technologies have been proposed. Arequirement needed for realizing high speed transmission rate radiocommunication is to increase frequency utilization efficiency. Since thetransmission rate and a bandwidth used are in a direct proportionalrelationship, a simple solution to increase the transmission rate is tobroaden the frequency bandwidth to be used. However, frequency bandsthat can be used are becoming scarcer and it is therefore unlikely thatsufficient bandwidth be assigned for constructing a new radiocommunication system. Consequently, it becomes necessary to increasefrequency utilization efficiency. In addition, another requirement is toseamlessly provide services in a private area (isolated cell) such as awireless LAN while realizing services in a communication area composedof cells such as mobile phones.

A technology that has a potential for meeting these requirementsincludes one-cell repetition OFDMA (Orthogonal Frequency DivisionMultiple Access). In this technology, communication is performed byusing the same frequency band in all cells in a communication areacomposed of these cells, and a modulation system for performingcommunication is OFDM. This communication method can realize faster datacommunication while isolated cells have a radio interface common to thatof a cell area as a matter of course.

An essential technology OFDM of the OFDMA will be described below. TheOFDM system is used in IEEE802.11a, which is a 5 GHz-band radio system,and Digital Terrestrial Broadcasting. The OFDM system arranges severaltens to several thousands of carriers at theoretically minimum frequencyintervals with no interference for simultaneous communications. In theOFDM, these carriers are usually called subcarriers and each subcarrieris modulated by a digital system such as PSK (phase shift modulation)and QAM (quadrature amplitude modulation) for communication. Further,the OFDM is said to be a frequency-selective fading resistant modulationsystem in combination with an error correction system.

A circuit configuration for modulation and demodulation will bedescribed using diagrams. Here, it is assumed that 768 subcarriers areused for the OFDM for a concrete description below.

FIG. 6 is a block diagram illustrating a schematic configuration of amodulation circuit of the OFDM. The modulation circuit shown in FIG. 6includes an error correction coding part 501, a serial to parallelconversion part (S/P conversion part) 502, a mapping part 503, an IFFTpart 504, a parallel to serial part (P/S conversion part) 505, a guardinterval insertion part 506, a digital to analog conversion part (D/Aconversion part) 507, a radio transmission part 508, and an antenna 509.Error correction encoding of information data to be transmitted isperformed by the error correction coding part 501. If a modulationscheme of each carrier is QPSK (four-phase modulation), 2×768=1536 bitsare output from an error correction coding circuit to generate one OFDMsymbol. Then, 2 bits are input into the mapping part 503 at a time fromthe S/P conversion part 502 as 768-system data, and modulation isperformed by the mapping part 503 for each carrier. Then, the IFFT part504 performs IFFT (Inverse Fast Fourier Transform). The number of pointsof the IFFT usually used for generating a 768-subcarrier OFDM signal is1024.

Data is allocated to f(n) (n is an integer between 0 and 1023) by themapping part and thus the IFFT part 504 will output data t(n). Sinceonly 768 pieces of data are input for 1024-point IFFT input in thepresent example, zero (both real and imaginary parts) is input as otherpieces of data. Normally, f(0) and f(385) to f(639) correspond to inputof zero. Then, after the data is converted to serial data by the P/Sconversion part 505, guard intervals are inserted by the guard intervalinsertion part 506. Guard intervals are inserted for reducinginterference between symbols when receiving an OFDM signal. If no guardinterval is used, IFFT output t(n) is output in order of t(0), t(1), . .. , t(1023) and these form symbols of the OFDM. When guard intervals areused, a latter half part of IFFT output will be output in accordancewith a guard interval length. If the guard interval length is ⅛ of anormal OFDM symbol, t(n) will be output in order of t(896), t(897), . .. , t(1023), t(0), t(1), . . . , t(1023). Then, after the data isconverted to an analog signal by the D/A conversion part 507, the analogsignal is converted to a frequency to be used for transmission, and thenthe data is transmitted from the antenna 509.

FIG. 7 illustrates a schematic view of spectrum of an OFDM signal afterD/A conversion, a schematic view of time waveforms after D/A conversion,and a schematic view after frequency conversion of the spectrum to atransmission band. f(n) and t(n) in FIG. 7 are the same as those shownin the above description.

It is known that if, usually when transmitting or receiving an OFDMsignal, the center of all bands is handled as DC in base-bandprocessing, a sampling frequency of an A/D converter and D/A converterwill be the smallest and also efficient. However, in the OFDM, as shownabove, no data is usually allocated to a DC component, that is, acarrier corresponding to f(0). Thus, power of the DC component is alsodepicted as zero in FIG. 7. It is obviously theoretically possible tomodulate the DC component, but the DC component is susceptible to noise(an influence of offset in the DC component of a circuit) in atransmitter or receiver and thus degradation of characteristics thereofis severe compared with other subcarriers. For this reason, almost allsystems do not modulate the subcarrier of the DC component.

Japanese Patent Application Laid-Open No. Hei 10-276165 and JapanesePatent Application Laid-Open No. Hei 11-154925, for example, describe aninfluence of the DC offset and how to eliminate the DC offset.

FIG. 8 is a block diagram illustrating the schematic configuration of anOFDM demodulator circuit. Basically, an operation that is opposite tothat performed by a transmission part is performed by a reception part.The demodulator circuit shown in FIG. 8 includes an error correctiondecoding part 701, a parallel to serial conversion part (S/P conversionpart) 702, a propagation path estimation demapping part 703, an FFT part704, a serial to parallel (P/S conversion part) 705, a guard interval(GI) removal part 706, an OFDM symbol synchronization part 707, ananalog to digital conversion part (A/D conversion part) 708, a radioreception part 709, and an antenna 710. Frequencies of radio wavesreceived by the antenna part 710 are converted down to frequency bandswhere A/D conversion can be performed by the radio reception part 709.

OFDM symbol synchronization of data converted to a digital signal by theA/D conversion part 708 is carried out by the OFDM symbolsynchronization part 707. Symbol synchronization is to determineboundaries of OFDM symbols from continuously incoming data. Data whosesymbol synchronization has been carried out is represented by t′(n). Ifthere is neither multipath nor noise in communication at all, t′(n)=t(n)holds. Guard intervals are removed by the guard interval removal part706. Therefore, after guard intervals are removed, t′(m) (m is aninteger between 0 and 1023) will be extracted. Then, parallel conversionof the data into 1024 pieces of data is performed by the S/P conversionpart 705. Then, 1024-point FFT (Fast Fourier Transform) is performed bythe FFT part 704 before f′(m) is output to the propagation pathestimation demapping part 703. However, since no modulation has beenperformed for m=0 and m=385 to 639 for transmission, f′(m) correspondingto such m are not input into the demapping part. Demodulation ofsubcarriers including propagation path estimation of 768 subcarriers isperformed by the propagation path estimation demapping part 703. Thedata is converted to serial data by the P/S conversion part 702 anderror corrections are carried out by the error correction decoding part701 before demodulation of transmission data is completed.

Next, the OFDMA will be described based on the above OFDM. The OFDMAsystem forms two-dimensional channels on frequency and time axes,arranges slots for communication two-dimensionally in a frame, andallows a mobile station to access a base station using the slots. FIG. 9is a diagram illustrating a two-dimensional frame configuration of theOFDMA. In this diagram, the vertical axis is the frequency axis and thehorizontal axis is the time axis. One rectangle is a slot used for datatransmission and a rectangle with oblique lines is a control slot usedby the base station to transmit broadcast information to all mobilestations. This diagram indicates that one frame has nine slots in a timedirection and twelve slots in a frequency direction, and 108 slots(among 108 slots, twelve slots are control slots) exist in total.Formally, a slot is represented by (Ta, Fb), with a time axis directionslot Ta (a is a natural number between 1 and 9) and a frequency axisdirection slot Fb (b is a natural number between 1 and 12). A shadedslot in FIG. 9, for example, is represented by (T4, F7).

In the present specification, twelve slots configured in the frequencydirection are called time channels and nine slots configured in the timedirection are called frequency channels or sub-channels.

Subcarriers of the OFDM will be divided and allocated to the frequencychannels. Since it is assumed that the OFDM has 768 subcarriers, 64subcarriers are allocated to each channel if divided equally amongtwelve slots. Here, it is assumed that subcarriers are allocated inincreasing order of spectrum in bands used for actual communication forconvenience and thus subcarriers f640 to f703 are allocated to F1,subcarriers f704 to f767 to F2, . . . , subcarriers f960 to f1023 to F6,subcarriers f1 to f64 to F7, subcarriers f65 to f128 to F8, . . . , andsubcarriers f321 to f384 to F12.

Communication from a base station (AP) to a mobile station (MT) will beconsidered. Many cases can be considered when the AP allocates data for15 slots to the MT and it is assumed here that data is allocated toslots with vertical lines in FIG. 9. That is, data to be received by theMT will be allocated to (T2 to T4, F1), (T5 to T8, F4), and (T2 to T9,F11). It is also necessary to embed data indicating allocation of datain a control slot corresponding to the frequency to be used to indicatethat the AP has allocated data to the MT. For the present example, (T1,F1), (T1, F4), and (T1, F11) correspond to such control slots.

The OFDMA system, based on what has been described above, allows aplurality of mobile stations to transmit and receive data to and fromthe base station by changing the frequencies and times. FIG. 9illustrates a gap between slots for convenience, but whether or notthere is a gap is not so important.

FIG. 10 is a block diagram illustrating a schematic configuration of aradio transmitter used for the OFDMA, and FIG. 11 is a block diagramillustrating the schematic configuration of a receiving circuit used forthe OFDMA. A transmitting circuit shown in FIG. 10 has a datamultiplexing part 901, and is divided into an error correction codingpart 902, an S/P conversion part 903, and a mapping part 904 for thenumber of channels (one to twelve). An IFFT part 905, a P/S conversionpart 906, a GI insertion part 907, a D/A conversion part 908, a radiotransmission part 909, and an antenna 910 fulfill functions similar tothose of the IFFT part 504, parallel to serial conversion part (P/Sconversion part) 505, guard interval insertion part 506, digital toanalog conversion part (D/A conversion part) 507, radio transmissionpart 508, and antenna 509 shown in FIG. 6 respectively.

In FIG. 10, the data multiplexing part 901 demultiplexes informationdata to be transmitted into twelve series in units of packets. That is,the multiplexer 901 physically specifies slots of the OFDMA specified bymodules such as CPU (not shown). Then, error correction encoding isperformed by the as many error correction coding parts 902 as thechannels, the data is demultiplexed into 64-system data by the as manyS/P conversion parts 903 as the channels, and modulation is performed bythe as many mapping parts 904 as the channels for each carrier beforeIFFT processing is performed by the IFFT part 905. Operations thereafterare the same as those described with reference to FIG. 6.

A receiving circuit shown in FIG. 11 has a data multiplexing part 101,and is divided into an error correction coding part 102, a parallel toserial conversion part (P/S conversion part) 103, and a propagation pathestimation demapping part 104 for the number of channels (one totwelve). An FFT part 106, a GI removal part 107, a synchronization part108, an A/D conversion part 109, a radio receiving part 110, and anantenna part 111 fulfill functions similar to those of the FFT part 704,serial to parallel conversion part (S/P conversion part) 705, guardinterval (GI) removal part 706, OFDM symbol synchronization part 707,analog to digital conversion part (A/D conversion part) 708, radioreception part 709, and antenna 710. Similar to the receiving circuitshown in FIG. 8, FFT processing is performed for received radio waves,and each of the twelve series of data undergoes propagation pathestimation, demapping, and error correction processing before beinginput into the data multiplexing part 101. Information data is processedby the data multiplexing part 101 before being output.

Modulation and demodulation processing shown here is only an example.Particularly, as many blocks as the number of channels, that is, twelveblocks are shown, but the present invention is not limited to thisnumber. Japanese Patent Application Laid-Open No. Hei 11-346203described a basic configuration of an OFDMA transmission apparatus.

-   Japanese Patent Application Laid-Open No. 10-276165-   Japanese Patent Application Laid-Open No. 11-154925-   Japanese Patent Application Laid-Open No. 11-346203

DISCLOSURE OF THE INVENTION

When the OFDMA is used for communication, terminals with variouscapabilities may be connected as a mobile station. One of such terminalsis a low power consumption terminal. This type of terminal isconstructed so as to reduce power consumption to be more suitable forportability even at the expense of a certain amount of transmission andreception capabilities. A method that may reduce power consumption of anOFDMA terminal is to narrow bandwidths that are capable of transmittingand receiving radio waves to limit accessible frequency channels.Limiting accessible frequency channels has disadvantages such as areduced transmission rate and not being able to select channels in goodpropagation condition, but also has advantages such as being able tolessen a processing speed, for example, a sampling frequency of an A/Dconverter and the processing speed of logic, and as a result, lowerpower consumption can be achieved.

Conventional OFDMA transmitters and receivers assume that, as describedabove, a receiving terminal receives and processes all bands. Thus, atransmitter adopts a system in which a subcarrier of a DC component(f(0)), being a center of all bands, is not used. The case where aterminal capable of receiving only one band in a state described abovemakes access will be discussed. Such a terminal filters a band to bereceived using an analog filter. If, for example, only the slot of F2(subcarrier numbers f(65) to f(128)) in FIG. 9 should be received, F2 isextracted by filtering and the center of this band, f(96) or f(97), willbe handled as a center frequency. Incidentally, selection of f(96) andf(97) shown here has no special meaning.

Since modulation has conventionally been performed for such subcarrierslike other subcarriers in a transmitter, a receiving terminal mustdemodulate such subcarriers despite bad characteristics. Thus, therehave been problems such as degraded characteristics, an occurrence oferrors in receiving slots, and an occurrence of retransmission, leadingto reduced throughput of an overall system. Such problems are notlimited to the terminal capable of receiving only one band, as describedabove, and concern various terminals, for example, those terminalscapable of receiving only two bands.

The present invention has been made in view of circumstances describedabove and an object thereof is to provide a radio transmitter capable ofperforming radio transmission without having an influence of an offsetof a DC component even to a communication party whose bandwidth that canbe used for transmission and reception is limited.

(1) To achieve the above object, the present invention has taken stepsshown below. That is, a communication method according to the presentinvention is a communication method in which a plurality of differentterminals performs communication using OFDM signals at the same time,wherein a transmitting terminal allocates minimum transmission power fortransmission to a specific subcarrier mutually known between thetransmitting terminal and a receiving terminal within a communicationslot, which is a frequency band in units of access, and the receivingterminal performs frequency conversion of a received signal assumingthat a frequency of the specific subcarrier corresponds to a directcurrent potential and converts the frequency-converted received signalto a digital signal by using an analog to digital converter for datademodulation.

Thus, the minimum transmission power is allocated for transmission to aspecific subcarrier mutually known between the transmitting terminal andreceiving terminal within a communication slot, which is a frequencyband in units of access, and therefore, radio transmission can beperformed without causing any influence of offset by a DC componentregardless of which bandwidth a communication party uses. Accordingly,it becomes possible to prevent deterioration of communicationcharacteristics and occurrence of errors in receiving slots to avoiddegradation of throughput because the DC component will not exert anyinfluence upon transmission and reception processing even ifcommunication is performed with a terminal whose bandwidth in use islimited in order to reduce power consumption.

(2) Also, a communication method according to the present invention is acommunication method in which a plurality of different terminalsperforms communication using OFDM signals at the same time, wherein areceiving terminal performs frequency conversion of a received signaland notifies a transmitting terminal of information about whether or nota frequency of a subcarrier corresponding to a direct current potentialabout the received signal which was inputted to an analog to digitalconverter can be used for data communication, and the transmittingterminal allocates, when the notified information indicates that thefrequency of the subcarrier corresponding to the direct currentpotential cannot be used for data communication, minimum transmissionpower to the subcarrier for transmission.

If information notified from the receiving terminal indicates that thefrequency of the subcarrier corresponding to the direct currentpotential cannot be used for data communication, the transmittingterminal allocates the minimum transmission power to the subcarrier, asdescribed above, and therefore, radio transmission can be performedwithout causing any influence of offset by a DC component at thereceiving terminal. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission and receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(3) Also, a communication method according to the present invention is acommunication method in which a plurality of different terminalsperforms communication using OFDM signals at the same time, wherein areceiving terminal performs frequency conversion of a received signaland notifies a transmitting terminal of information about whether or nota frequency of a subcarrier corresponding to a direct current potentialabout the received signal which was inputted to an analog to digitalconverter can be used for data communication, and the transmittingterminal allocates minimum transmission power to a subcarrier of thenotified frequency for transmission.

Thus, the transmitting terminal allocates the minimum transmission powerfor transmission to the subcarrier of the frequency notified from thereceiving terminal, and therefore, radio transmission can be performedwithout causing any influence of offset by a DC component at thereceiving terminal. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission/receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(4) Also, the communication method according to the present invention ischaracterized in that the minimum transmission power is zero.

Thus, the minimum transmission power is zero, and therefore, radiotransmission can be performed without causing any influence of offset bya DC component. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission/receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(5) Also, the communication method according to the present invention ischaracterized in that the specific subcarrier mutually known between thetransmitting terminal and receiving terminal is a center frequency ofthe communication slot.

Thus, the known specific subcarrier is the center frequency of thecommunication slot, and therefore, an influence of offset by a DCcomponent can be avoided by allocation of the center frequency of thecommunication slot to the DC component in reception processing by thereceiving terminal. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission/receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(6) Also, the communication method according to the present invention ischaracterized in that the specific subcarrier mutually known between thetransmitting terminal and receiving terminal is one of a maximumfrequency and a minimum frequency of the communication slot.

Thus, the known specific subcarrier is one of the maximum frequency andminimum frequency of the communication slot, and therefore, it becomespossible to easily determine the subcarrier to be a DC component or thesubcarrier corresponding to the center frequency in bandwidths used bythe receiving terminal. That is, if there are even subcarriers includedin the communication slot, the subcarrier corresponding to the centerfrequency can be determined by making the number of subcarriers oddafter excluding (allocating no modulated data to) the subcarriercorresponding to the maximum frequency or minimum frequency. Since it issill possible to allocate no modulated data to the subcarrier to be a DCcomponent or the subcarrier corresponding to the center frequency evenif a plurality of frequency channels is used by excluding (allocating nomodulated data to) the subcarrier corresponding to the maximum frequencyor minimum frequency, radio transmission can be performed withoutcausing any influence of offset by a DC component regardless of whichbandwidth a communication party uses. Thus, it becomes possible toprevent deterioration of communication characteristics and occurrence oferrors in receiving slots to avoid degradation of throughput.

(7) Also, the communication method according to the present invention ischaracterized in that the transmitting terminal does not allocateinformation data to a subcarrier to which the minimum transmission poweris allocated.

Since no information data is allocated to the subcarrier to which theminimum transmission power is allocated, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput.

(8) Also, a radio transmitter according to the present invention is aradio transmitter applied to an OFDMA communication system in which aplurality of different terminals performs communication using OFDMsignals at the same time, the transmitter comprises: a mapping part thatallocates transmission power to each subcarrier, and also selects asubcarrier to which minimum power of the transmission power to beallocated is allocated, and modulates transmission data in units ofcommunication slots to output the modulated data; and a transmissionpart for transmitting radio signals including the modulated data usingeach of the subcarriers.

Thus, the subcarrier to which the minimum transmission power oftransmission power to be allocated is selected, and therefore, itbecomes possible to select a specific subcarrier known between atransmitting terminal and a receiving terminal, select a subcarrier thatcannot be used for data communication, and a subcarrier notified fromthe receiving terminal. As a result, radio transmission can be performedwithout causing any influence of offset by a DC component regardless ofwhich bandwidth a communication party uses. Accordingly, it becomespossible to prevent deterioration of communication characteristics andoccurrence of errors in receiving slots to avoid degradation ofthroughput because the DC component will not exert any influence upontransmission and reception processing even if communication is performedwith a terminal whose bandwidth in use is limited in order to reducepower consumption.

(9) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part allocates zero to the selectedsubcarrier as the transmission power.

Thus, zero is allocated to the selected subcarrier as transmissionpower, and therefore, radio transmission can be performed withoutcausing any influence of offset by a DC component. Accordingly, itbecomes possible to prevent deterioration of communicationcharacteristics and occurrence of errors in receiving slots to avoiddegradation of throughput because the DC component will not exert anyinfluence upon transmission/reception processing even if communicationis performed with a terminal whose bandwidth in use is limited in orderto reduce power consumption.

(10) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part selects a subcarriercorresponding to a center of a communication slot.

Thus, the subcarrier corresponding to the center of the communicationslot is selected, and therefore, an influence of offset by a DCcomponent can be avoided by allocation of the center frequency of thecommunication slot to the DC component in reception processing by thereceiving terminal. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission and receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(11) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part selects a subcarriercorresponding to a maximum frequency or a minimum frequency of acommunication slot.

Thus, the subcarrier corresponding to the maximum frequency or minimumfrequency of the communication slot, and therefore, it becomes possibleto easily determine the subcarrier to be a DC component or thesubcarrier corresponding to the center frequency in bandwidths used bythe receiving terminal. That is, if there are even subcarriers includedin the communication slot, the subcarrier corresponding to the centerfrequency can be determined by making the number of subcarriers oddafter excluding (allocating no modulated data to) the subcarriercorresponding to the maximum frequency or minimum frequency. Since it issill possible to allocate no modulated data to the subcarrier to be a DCcomponent or the subcarrier corresponding to the center frequency evenif a plurality of frequency channels is used by excluding (allocating nomodulated data to) the subcarrier corresponding to the maximum frequencyor minimum frequency, radio transmission can be performed withoutcausing any influence of offset by a DC component regardless of whichbandwidth a communication party uses. Thus, it becomes possible toprevent deterioration of communication characteristics and occurrence oferrors in receiving slots to avoid degradation of throughput.

(12) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part selects a frequency of asubcarrier corresponding to a direct current potential only ifsubcarrier availability information notified from a communication partyindicates that the frequency cannot be used for data communication.

Thus, only if subcarrier availability information notified from acommunication party indicates that the frequency of a subcarriercorresponding to a direct current potential cannot be used for datacommunication, the subcarrier is selected, and therefore, radiotransmission can be performed without causing any influence of offset bya DC component on the communication party. Accordingly, it becomespossible to prevent deterioration of communication characteristics andoccurrence of errors in receiving slots to avoid degradation ofthroughput because the DC component will not exert any influence upontransmission and reception processing even if communication is performedwith a terminal whose bandwidth in use is limited in order to reducepower consumption.

(13) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part selects a frequency notified froma communication party.

Thus, the subcarrier of the frequency notified from a communicationparty is selected, and therefore, radio transmission can be performedwithout causing any influence of offset by a DC component on thecommunication party. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission and receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

(14) Also, the radio transmitter according to the present invention ischaracterized in that the mapping part updates a subcarrier frequency tobe selected each time a communication party with which communication isperformed using communication slots changes.

Thus, the subcarrier frequency to be selected is updated each time acommunication party with which communication is performed changes, andtherefore, processing in accordance with the communication party can beperformed. Radio transmission can thereby be performed without causingany influence of offset by a DC component regardless of which bandwidtha communication party uses. Accordingly, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission and receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

According to the present invention, it becomes possible to preventdeterioration of communication characteristics and occurrence of errorsin receiving slots to avoid degradation of throughput because the DCcomponent will not exert any influence upon transmission and receptionprocessing even if communication is performed with a terminal whosebandwidth in use is limited in order to reduce power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a schematic configuration of atransmitting circuit according to a first embodiment.

FIG. 2 is a diagram illustrating allocation of communication slots insome frame.

FIG. 3 is a diagram illustrating unused subcarrier numbers in each timeslot.

FIG. 4 is a block diagram illustrating the schematic configuration of atransmitting circuit according to a second embodiment.

FIG. 5 is a flow chart illustrating operations of an unused subcarrieroperation part 11.

FIG. 6 is a block diagram illustrating the schematic configuration of aconventional OFDM modulation circuit.

FIG. 7 illustrates a schematic view of spectrum of an OFDM signal afterD/A conversion, a schematic view of time waveforms after D/A conversion,and a schematic view after frequency conversion of the spectrum to atransmission band.

FIG. 8 is a block diagram illustrating the schematic configuration of aconventional OFDM demodulator circuit.

FIG. 9 is a diagram illustrating a two-dimensional frame configurationof a conventional OFDMA.

FIG. 10 is a block diagram illustrating the schematic configuration of atransmitting circuit used for the conventional OFDMA.

FIG. 11 is a block diagram illustrating the schematic configuration of areceiving circuit used for the conventional OFDMA.

BEST MODES FOR CARRYING OUT THE INVENTION

Radio communication systems according to present embodiments will bedescribed below. The present embodiments assume a communication systembased on the above OFDMA.

The present embodiments only exemplify circuit configurations andcontrol methods, and purposes thereof are not to modulate a subcarriercorresponding to a DC component in a radio transmitter to avoid anyinfluence of noise of the DC component in a transmitting circuit andsimilarly not to demodulate the subcarrier corresponding to the DCcomponent in a receiving circuit. Thus, there are various methodsavailable for implementation.

First Embodiment

In a first embodiment, a terminal is shown in which, regardless of whichbandwidth a terminal connected is capable of processing, no modulateddata is provided to a subcarrier selected as a center frequency by theterminal. In a conventional technology, relationships betweensub-channels and subcarriers are: subcarriers f(640) to f(703) allocatedto F1, subcarriers f(704) to f(767) to F2, . . . , subcarriers f(960) tof(1023) to F6, subcarriers f(1) to f(64) to F7, subcarriers f(65) tof(128) to F8, . . . , and subcarriers f(321) to f(384) to F12, but heresubcarriers whose subcarrier number exceeds 512 are represented bysubtracting 1024. Thus, new representations will be changed to:subcarriers f(-384) to f(-321) allocated to F1, subcarriers f(-320) tof(-257) to F2, . . . , subcarriers f(-64) to f(-1) to F6, subcarriersf(1) to f(64) to F7, subcarriers f(65) to f(128) to F8, . . . , andsubcarriers f(321) to f(384) to F12.

FIG. 1 is a block diagram illustrating a schematic configuration of atransmitting circuit according to the first embodiment. The transmittingcircuit shown in FIG. 1 has a data multiplexing part 1, and is dividedinto an error correction coding part 2, an S/P conversion part 3, and amapping part for the number of channels (one to twelve). An IFFT part 5,a P/S conversion part 6, a GI insertion part 7, a D/A conversion part 8,a radio transmission part 9, and an antenna part 10 fulfill functionssimilar to those of the IFFT part 504, parallel/serial conversion part(P/S conversion part) 505, guard interval insertion part 506,digital/analog conversion part (D/A conversion part) 507, radiotransmission part 508, and antenna 509 shown in FIG. 6 respectively.

The mapping part 4 allocates transmission power to each subcarrier andalso selects a subcarrier to which minimum power (for example, zero) ofthe transmission power to be allocated should be allocated. Then,transmission data is modulated in units of communication slots and themodulated data is output. In the mapping part 4 described above, eachcorresponding sub-channel number has been added and marking of f(m) hasbeen changed to m=−512 to 511. In the conventional technology,modulation of subcarriers corresponding to the subcarrier numbers zero,385 to 511, and −385 to −512 is not performed. In the first embodiment,in addition, modulation of subcarriers corresponding to the subcarriernumbers 32×p (p is an integer between −12 and 12) is not performed.Viewed from slot allocation, this means that the number of subcarriersused by each sub-channel is 62 and subcarriers in the center of eachsub-channel and between sub-channels are not modulated.

A terminal that can receive only one sub-channel filters the onesub-channel to perform reception processing. Since in this case nomodulation of subcarrier in the center of each sub-channel has beenperformed, data can be demodulated without deterioration ofcharacteristics by ignoring the center for demodulation like aconventional OFDM receiver. Similarly, since the center frequency of aterminal that can access only x (x is an odd number equal to 12 orsmaller) sub-channels will be the center of a sub-channel under thecurrent assumption and the subcarrier thereof is not used formodulation, data can be demodulated without deterioration ofcharacteristics by ignoring the center for demodulation like theconventional OFDM receiver.

The center of a terminal that can access only y (y is an even numberequal to 12 or smaller) sub-channels will be between sub-channels. Sincealso a subcarrier between sub-channels is not used for modulation,similar to the conventional OFDM receiver, data can be demodulatedwithout deterioration of characteristics by ignoring the center fordemodulation.

In the first embodiment, as described above, receivers suitable forvarious bands can be connected without deterioration of characteristics.

Second Embodiment

In the first embodiment described above, a method was shown in whichsubcarriers not to be used are selected in advance to deal with variousterminals. However, according to this method, the transmission rate of ahighly capable terminal that can use all bands for transmission andreception may be lower than that of a conventional method. Subcarriersthat cannot be used are set in the first embodiment while all 768subcarriers can be used in the conventional method, and therefore, thenumber of available subcarriers is 744 and, if an identical modulationis applied to all subcarriers, the rate thereof will drop to 744/768.

Thus, in the second embodiment, a method in which subcarriers not usedadaptively are set will be described.

FIG. 2 is a diagram illustrating allocation of communication slots insome frame. Similar to the conventional technology, slots with obliquelines are broadcast slots received by all terminals and the terminals Ato F perform communication using indicated slots respectively. Whendetermining a center subcarrier position in descriptions below,processing is performed by assuming that the number of subcarriers usedis odd so that the processing can be made easier to understand. However,there is no inevitability for this assumption and, if an even number ofsubcarriers are used for processing, the center frequency will be afrequency at which no subcarrier exists and no problem will be caused byarranging in advance which subcarrier to use as the center subcarrierbetween the transmitting and receiving apparatuses.

Since the control slots need to be received by all stations in FIG. 2,similar to the first embodiment, subcarriers not used for modulation arearranged. More specifically, the subcarrier numbers not used formodulation are zero, 385 to 511, −385 to −512, and 32×p (p is an integerbetween −12 and 12).

Next, focusing on A, slots to be used are five slots of (T2 to T6, F12)and the frequency channel is F12 only. f(321) to f(384) are allocated toF12 and it is assumed that the subcarrier with the maximum number f(384)and the subcarrier f(352) positioned in the center after excludingf(384) are not to be used.

Focusing on B, slots to be used are nine slots of (T2, F7 to F9) and (T5to T6, F7 to F9). Subcarriers to be used for F7 to F9 are f(1) to f(192)and it is assumed that the subcarrier with the maximum number f(192) andthe subcarrier f(96) positioned in the center after excluding f(192) arenot to be used.

C uses 10 slots of (T3, F1 to F10). Subcarriers to be used are f(-384)to f(256). If subcarriers to be accessed sandwich f(0), processing notto use a subcarrier with the maximum number is not performed. Thus, onlythe subcarrier f(-64) positioned in the center is not to be used. f(0)is naturally not used.

D uses 18 slots of (T2, F1 to F6) and (T4 to T5, F1 to F6). Subcarriersto be used are f(-384) to F(-1). Thus, the subcarrier f(-1) with themaximum number and the subcarrier f(-193) positioned in the center arenot to be used.

E uses 4 slots of (T4 to T5, F10 to F11). Subcarriers to be used aref(193) to F(320). Thus, the subcarrier f(320) with the maximum numberand the subcarrier f(256) positioned in the center are not to be used.

F uses 36 slots of (T7 to T9, F1 to F12). Subcarriers to be used aref(-384) to F(384). Only the subcarrier f(0) positioned in the center isnot to be used.

FIG. 3 summarizes unused subcarriers in units of time slots. As isevident from FIG. 3, the number of unused subcarriers has decreased incomparison with the first embodiment and terminals capable of accessingall bands can use exactly as many subcarriers as before. Continuousbands are allocated to slots in FIG. 2 and even if an unused slot ispresent therebetween, no problem will be caused by performing processingunder the assumption that a band thereof is being used.

FIG. 4 is a block diagram illustrating the schematic configuration of atransmitting circuit according to the second embodiment. When comparedwith the transmitting circuit according to the first embodiment shown inFIG. 1, the transmitting circuit according to the second embodimentadditionally has the unused subcarrier operation part 11. The unusedsubcarrier operation part 11 carries out a function to operate unusedsubcarriers described above. The slot number, the terminal ID using theslot, and the maximum number and minimum number of the sub-channelnumber to be used are input into the unused subcarrier operation part11.

FIG. 5 is a flow chart illustrating operations of the unused subcarrieroperation part 11. Parameters used in FIG. 5 are the same as thosedescribed above. However, fdc is an index value showing whether or not achannel used contains a DC component, TS is a variable value of the slotnumber, and m_max and m_min are the maximum value and minimum value ofthe sub-channel to be input into the unused subcarrier operation part 11for use respectively. An unused subcarrier is represented as f(m)=0.

When starting to configure a frame in S101, f(0), f(-385 to -512), andf(-385 to -512) are set always to zero. Also, fdc=0 and TS=0 are set. InS102, TS is incremented by one. In S103, whether the current slot is abroadcast slot is determined. Since in the present embodiment broadcastinformation is transmitted using the T1 slot, the slot is determined tobe a broadcast slot if TS=1. If the slot is a broadcast slot, f(m) withm=32×p (p is an integer between −12 and 12) for subcarriers not to betransmitted is set to zero in S104.

If TS is equal to or greater than 2, proceed to S105. Here, whether inapplicable TS there is a terminal to which a slot should be allocated isdetermined. If there is such a terminal, proceed to S106, and if thereis no such terminal, proceed to S110. In S106, an fdc operation isperformed. fdc is an operation based on the subcarrier number. In S107,whether sub-channels are allocated by sandwiching f(0) is determinedbased on the value of fdc. If fdc is negative, proceed to S109 becausesub-channels are allocated by sandwiching f(0). If fdc is positive,proceed to S108. S108 is a process to determine unused subcarriers whenf(0) is not sandwiched and the subcarrier f(m_max) with the maximumvalue and the center subcarrier f(m_max+m_min−1) in the band excludingf(m_max) are set to zero respectively.

S109 is a process to determine unused subcarriers when f(0) issandwiched and the subcarrier f(m_max+m_min) to be the center in theband is set to zero. In S110, whether slots have been allocated up to aframe end is determined. Since in the second embodiment the time slot isup to 9, whether TS=9 or not is determined. If TS=9, processing isterminated to return to an initial state.

By determining unused subcarriers according to the method describedabove for each frame, communication can be performed efficiently withoutsuffering degradation of characteristics.

Unused subcarriers are determined in the first and second embodimentsunder the assumption that an influence of DC noise of a receptionapparatus is always considerable, but existence of a terminal with verygood characteristics can also be considered. Thus, the introduction of afunction to determine unused subcarriers to eliminate an influence ofthe DC noise in the reception apparatus can also be considered when arequest is made from a terminal.

That is, if notification that any subcarrier to be a DC component in allfrequency channels of allocated communication slots cannot be used isreceived from a terminal, no modulated data is allocated to thesubcarrier and therefore, it becomes possible to prevent deteriorationof communication characteristics and occurrence of errors in receivingslots to avoid degradation of throughput with a communication party inwhich communication characteristics of a subcarrier to be a DC componentare degraded. For a communication party in which communicationcharacteristics of a subcarrier to be a DC component are not degraded,on the other hand, it becomes possible to increase utilizationefficiency of frequencies by allocating modulated data also to thesubcarrier to be a DC component.

Though examples in which the numbers of subcarriers of basicsub-channels are identical in all sub-channels are shown for both thefirst and second embodiments, these are only basic examples and can alsobe applied easily when the numbers of subcarriers are different indifferent sub-channels.

Incidentally, base station equipment can be configured by a transmittingcircuit according to the present embodiments. With this base stationequipment, it becomes possible to prevent deterioration of communicationcharacteristics and occurrence of errors in receiving slots to avoiddegradation of throughput because the DC component will not exert anyinfluence upon transmission and reception processing even ifcommunication is performed with a terminal whose bandwidth in use islimited in order to reduce power consumption.

EXPLANATION OF NUMERALS

-   1: data multiplexing part-   2: error correction coding part-   3: S/P conversion part-   4: mapping part-   5: IFFT part-   6: P/S conversion part-   7: guard interval (GI) insertion part-   8: D/A conversion part-   9: radio transmission part-   10: antenna part-   11: unused subcarrier operation part

1. A communication method in which a plurality of different terminalsperforms communication using OFDM signals at the same time, wherein atransmitting terminal allocates minimum transmission power fortransmission to a specific subcarrier mutually known between thetransmitting terminal and a receiving terminal within a communicationslot, which is a frequency band in units of access, and the receivingterminal performs frequency conversion of a received signal assumingthat a frequency of said specific subcarrier corresponds to a directcurrent potential and converts the frequency-converted received signalto a digital signal by using an analog to digital converter for datademodulation.
 2. A communication method in which a plurality ofdifferent terminals performs communication using OFDM signals at thesame time, wherein a receiving terminal performs frequency conversion ofa received signal and notifies a transmitting terminal of informationabout whether or not a frequency of a subcarrier corresponding to adirect current potential about the received signal which was inputted toan analog to digital converter can be used for data communication, andsaid transmitting terminal allocates, when said notified informationindicates that the frequency of the subcarrier corresponding to thedirect current potential cannot be used for data communication, minimumtransmission power to said subcarrier for transmission.
 3. Acommunication method in which a plurality of different terminalsperforms communication using OFDM signals at the same time, wherein areceiving terminal performs frequency conversion of a received signaland notifies a transmitting terminal of information about whether or nota frequency of a subcarrier corresponding to a direct current potentialabout the received signal which was inputted to an analog to digitalconverter can be used for data communication, and said transmittingterminal allocates minimum transmission power to a subcarrier of saidnotified frequency for transmission.
 4. The communication methodaccording to any of claims 1 to 3, wherein said minimum transmissionpower is zero.
 5. The communication method according to claim 1, whereinsaid specific subcarrier mutually known between the transmittingterminal and receiving terminal is a center frequency of saidcommunication slot.
 6. The communication method according to claim 1,wherein said specific subcarrier mutually known between the transmittingterminal and receiving terminal is one of a maximum frequency and aminimum frequency of said communication slot.
 7. The communicationmethod according to any of claims 1 to 3, wherein said transmittingterminal does not allocate information data to a subcarrier to which theminimum transmission power is allocated.
 8. A radio transmitter appliedto an OFDMA communication system in which a plurality of differentterminals performs communication using OFDM signals at the same time,comprising: a mapping part that allocates transmission power to eachsubcarrier, and also selects a subcarrier to which minimum power of saidtransmission power to be allocated is allocated, and modulatestransmission data in units of communication slots to output themodulated data; and a transmission part for transmitting radio signalsincluding said modulated data using each of said subcarriers.
 9. Theradio transmitter according to claim 8, wherein said mapping partallocates zero to said selected subcarrier as the transmission power.10. The radio transmitter according to claim 8 or 9, wherein saidmapping part selects a subcarrier corresponding to a center ofcommunication slots.
 11. The radio transmitter according to claim 8 or9, wherein said mapping part selects a subcarrier corresponding to amaximum frequency or a minimum frequency of a communication slot. 12.The radio transmitter according to claim 8 or 9, wherein said mappingpart selects a frequency of a subcarrier corresponding to a directcurrent potential only if subcarrier availability information notifiedfrom a communication party indicates that the frequency cannot be usedfor data communication.
 13. The radio transmitter according to claim 8or 9, wherein said mapping part selects a frequency notified from acommunication party.
 14. The radio transmitter according to claims 8 or9, wherein said mapping part updates a subcarrier frequency to beselected each time a communication party with which communication isperformed using communication slots changes.
 15. A communication methodin which a plurality of different terminals performs communication usingOFDM signals at the same time, wherein a transmitting terminal allocatesminimum transmission power for transmission to specific subcarriersmutually known between the transmitting terminal and a receivingterminal within a communication slot in a frequency band, which is anaccess unit of the receiving terminal, and the receiving terminalselects a subcarrier from said specific subcarriers so that a receivingbandwidth falls within a system band, performs frequency conversionassuming that a frequency of said selected subcarrier corresponds to adirect current potential, and converts a frequency-converted receivedsignal to a digital signal by using an analog to digital converter fordata demodulation.
 16. A radio transmitter applied to an OFDMAcommunication system in which a plurality of different terminalsperforms communication using OFDM signals at the same time, comprising:a mapping part that allocates transmission power to each subcarrier, andalso selects a specific subcarrier within a communication slot in afrequency band, which is an access unit of a receiving terminal,allocates minimum transmission power to the selected subcarrier, andmodulates transmission data to output the modulated data; and atransmission part for transmitting radio signals including saidmodulated data using each of said subcarriers.